It's very difficult to brute force 6 words, but not beyond the realms of possibility. Here's an example of someone brute forcing the last 4 words of a 12 word seed phrase in a day:

https://medium.com/@johncantrell97/how-i-checked-over-1-trillion-mnemonics-in-30-hours-to-win-a-bitcoin-635fe051a752

Note: It's much easier to bruteforce the last word as this is just a checksum.

But the BEST reason not to do this is because it's just inferior to using a passphrase. Both are 2-of-2 schemes, but a passphrase is A) Easier to memorise, B) Gives you plausible deniability with the seed-only wallet and C) Is an industry standard supported by every hardware wallet.

Don't do it, having 6 of the words makes the last 6 feasible to brute force:

https://btctranscripts.com/andreas-antonopoulos/2020-04-08-andreas-antonopoulos-seed-splitting/